It's a sacoglossan sea slug, but hard to give you an exact ID from the photos. It looks like a Julia species with an abnormally small shell, or an Oxynoe species with an abnormally short tail! Or maybe it looks like a Phanerophthalmus species with unusually distinct rhinophores....I can't decide. A sacoglossan expert could probably at least give you a family-level ID, which I can't seem to do with any level of confidence. It could even be one of the stumpy Elysia species. Whichever it is, it will feed on algae, and won't hurt anything in your tank.
Cheers,
Don